Selective detector circuit



M. L. ALEXANDER SELECTIVE DETECTOR CIRCUIT June 27, 19,50

Filed March 27, 1947 FIGB INVENTOR MAURO L.ALEXANDER A MAX.

Patented June 27,v 1950 UNITED STATES PATENT OFFICE SELECTIVE DETECTOR CIRCUIT Maurice L. Alexander, Fort Wayne, lndsassignor, by mesne assignments, to Farnsworth Research Corporation, a corporation of Indiana Application March 27, 1947; Serial N o. 737,453

(Cl. Z50-20) 3'Claims. 1 This invention relates in general to radio receivers and more particularly to a detector circuit adapted to adjustably control the effective selectivity response of a receiver.

The present day radio receivers are provided.

-rad-io frequency transformer design, the number of transformers employed, and the degree of coupling practiced in the radio frequency stages which precede the detector. It is well known that a receiver-having a broad characteristic response issubject to noise and side band interference due to its inherent lack of selectivity.

Many methods have been advanced for sharpening the receiver response. However, it is well known that the sharpening of the characteristic response, as exemplified by the narrowing ofthe response curve skirt, usually also results in `the simultaneous sharpening of the peak of the curve. Thus the tuning'of the receiver becomes extremely sharp and critical, thereby requiring a visual tuning indicator Vfor tuning. At the same time, due to the narrowing of its band-pass characteristics, such a receiver is incapable of satisfactory reception and reproduction of the relatively nominal band width `of frequencies covered` by a carrier signal which may contain modulative componentssuch as produced byL musical instru- The prior art radio receiver expressly adapted for-selective operation represents a relativelyexpensive investment.v and such a receiver is usually not easily converted 0to broad band-pass .char- `acteristics without necessitating aV plurality of circuits and controls for such purpose. It is to beiurther` noted, thatl in a prior art type of receiver there is a :noticeable diierence, in tuning :dial band width coverage, between signals receivedfrom local or long distance stations.

Obviously in inexpensive radio receiver having an apparent characteristic response, the curve of which'would vnot slope out into-a wide skirt, but `which would assume-the -form y of a ysubstantially straightsided skirt, would be desirable by reason of being more selective, thus eliminating the aforementicnediside band; interference. Yet at thesa-metime.-therequired, or fairly broad', frequency response of the receiver should not -be detrimentally alected provided that theresponse curve peakr is not sharpened. Additional efficiency -may be further securedY by adjustably controlling the ,frequency width of this straightsided response to the requisite degree for. selectivity purposes orfor adequate audio frequency coverage. A further advantage would be realized by'arrangingl forthe insertion, zremoval, or variationv of the selectivitycontrolling circuit by the manipulation `of but asinglecontrol.

It is the purpose of thisinvention to ,remedy the aforementioned deficiencies of the prior art rreceivers and, at the Sametime, to accomplish this purpose by the novel use of but'a fewinexpensive vreceiver components.

Therefore, itis an object of the present invention tol providev a selective detector circuit adapted Yto provide a characteristic overall response having substantially straight sides.

lAnother object ofthe present invention is yto provide a receiverfof. `substantially quieter operation, whereby the effective narrowing of the characteristic response eliminates side band noises and other interferences attendant -.to broad frequency response.

Still another object ofthe present invention is to provide a receiver `which is relatively easy to tune to exact .resonance without necessitating a visual tuning indicator.

A further object of the present invention is to provide a detector circuit wherein the overall characteristicresponse may be `manually varied by a single control to a predetermined degree while still maintaining a straight sided characteristic response.

Yet another'object Aof the present invention is to provide aselective receiver wherein the reception of a=1ocal or vlong distancestation will be morefnearly .alike infband width: coverage yon the tuning dial, and more nearly alike inivolume level.

Still another object of the present invention is toprovide anormally broad `band-.pass receiver witha relativelyselective and :narrow band-pass detector circuit' which. may be instantlyrendered inoperative whereupon the receiver will Aimmediately be restored to its normal response characteristic. l

Inaccordance with this invention, in general, there is providedaifsource oiA an intermediate frequency wave which is modulated by an-intelligence signal. Also,:there isprovided-a-rst resonant circuit which is--coupled to the-.inter- .uk mediate frequency wave source and which is fied as a buckling voltage.

broadly tuned to the intermediate frequency. A second resonant circuit is coupled to the first resonant circuit and is narrowly tuned to the intermediate frequency. Additionally, there is provided means coupled to the second resonant circuit for demodulating the intermediate frequency Wave voltage which is developed in the second resonant circuit. Another means coupled to the first resonant circuit functions to demodulate the intermediate frequency wave voltage developed in the rst resonant circuit. There, furthermore, is provided means coupling the two demodulating means which functions to bias to an inoperative state Whichever one of said two demodulating means has impressed thereon the voltage of lesser magnitude.

In accordance with a particular embodiment of this invention there is provided a radio receiver arranged with conventional tuning facilities and having a rst rectiiier circuit and a second rectifier circuit. These rectifier circuits are provided with a signal input transformer having a pair of windings each of which is tuned to resonance at the same'frequency. A first winding is connected to a source of signals and has a relatively broad frequency response characteristic. This Winding is also coupled to said rst rectifier whereby a rectified signal voltage will be effected which shall be hereinafter classi- A second winding is coupled to the first Winding and has ya relatively narrow frequency response characteristic. This Winding is coupled to said second rectifier which normally functions as a detector stage to provide audio signal voltages and automatic volume control voltage.

Both rectifying Acircuits are arranged with a common cathode circuit whereby the aforementioned vbucking voltage may be applied to this cathode to exercise a controlling effect over the normal signal detector circuit. There is also provided in this cathode circuit a cathode resistor, of variable type, whereby control may be exercised -over the magnitude of bucking voltage which Imay be developed at and impressedV upon the cathode. The bucking voltage may thereupon be applied to the detector signal voltage and thereby effectivelyinfluencing the detector stage to apparently maintain a relatively narrow frequency response characteristic regardless of the amplitude of the applied inputsignals.

For a better understanding of the inventionr together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims. f

In the accompanying drawing; i

Fig. 1 is a partly schematic block diagram of a radio receiver provided with the variably selective detector circuit embodying the present invention; and

Fig. 2 and Fig. 3 illustrate the response curves of the receiver showing the normal characteristic response, in comparison with the straight sided selective response obtained with the present invention. I

Referring now to Fig. 1 of the drawing, there is shown a superheterodyne type of receiver which in most respects is conventional. Carrier' signal energy is collected by an antenna I and is impressed upon a radio frequency amplifier l2. There is also coupled to this amplifier a local oscillator |3 whereby the amplifier may function also asa frequency converter stage. The

aciacet 4 output of radio frequency amplifier I2 is coupled to the input of an intermediate frequency amplifier I4. Space current for the radio frequency amplifier, the intermediate frequency amplifier, and the local oscillator is provided by a connection to the high voltage conductor |5 which is terminated at a power supply I6. The output of the intermediate frequency amplifier i4 is coupled to the primary winding I8 of an intermediate frequency transformer I9. The primary transformer winding is tuned to the rintermediate frequency by a shunt-connected condenser 2n in such a manner that it normally has la relatively broad frequency response characteristie. The intermediate frequency transformer I9 also is provided with a secondary winding 2| which is inductively coupled to the primary windingI il and is shunted by a condenser 22 by means of which the winding 2| is tuned to be responsive to the intermediate frequency in a manner whereby its frequency response characteristic is relatively narrow as compared to that of primary winding I8.

There is coupled-to thesecondary Winding 2| a signal rectifying or detection circuit. This circuit includes the left hand portion of a duodiode 24, the anode 25 of which is connected to the upper terminal of the winding 2|, and the cathode 26 of which is connected toacathode condenser 2 and also to the movable arm 28 of a variable resistor 29 which has one end thereof connected to ground. The lower terminal of the secondary winding 2| is connected through a series arrangement of resistors 3| and A32 to ground. Condensers 33 and 34 are connected from the respective terminals of the resistor 3| to ground and Iserve to filter the intermediate carrier frequency component from the audio circuit. A resistor 36 is connected at the junction point between resistors 3I and 32 and is coupled by means of a conductor 3'! to the radio frequency amplifier I2 and to the intermediate frequency amplifier I4 to effect automatic volume control in the usual manner. A condenser 40 is conventionally coupledl between conductor 31 and ground for filtering-and removal of audio frequency components'from the automatic volume control circuit. A conductor 38 also is connected to the junction point between resistors 3| and 32 for applying an audio signal input voltage to an audio-amplifier 39. The output of the audio amplifier 39 is coupled to a loud speaker 4| for the audible reproduction of the audio signals. 'Space current for the audio amplifier stage 39 is provided by a conductor 42 which connects with the power supply I6.

. The radio frequency amplifier I2 may be tuned, and the frequency of the local oscillatoryl3 vmay be varied simultaneously therewith, v1in a Iconventional manner, by mechanically coupling the respective tuning elements by a linkage 43. -`These tuning elements may be controlled manually by means of a tuningl dial 44 connected vto the linkage. f

A bucking voltage rectifier circuit isderived from the primary winding I8 of the intermediate frequency transformer IS. This winding is coupled by acondenser 45 to the right hand anode 46 of the duo-diode 24. 'This rectifier circuit also includes a resistor A4'! which is connected between the anode 45 and ground. It will be noted that the primary rectifier circuit andthe secondary rectifier circuit employ a common cathode circuit. l i y Referring now 'to the normal operation of the receverf'and'-'its vdetector-Jstage, assume-thatthe cat lfiocleiresistor--ZQ-is adjusted to 'a value of Zero resiStaEi-icewhereby the :cathode 26 of Ctlfiadiocle f243is; laced-at ground potential. Carrier-'signal energywwhich -is collecteiby- Vthe antenna I I Y is conducted xto theinput -of Jthe radio frequency amplier I2.A The manipulation of A'the ltuning `dia-l iliwi-ll cause the linkage-43 y'to simultaneously mov 'the tuning elements associatedl withJ-th'e ampli-fier. l2--and the'os'cillator I`3-.- Whenl res,- on'ance with a-carrierA-signal is reached; the am'- plier I 2, functioning as a converter stage, causes generation of `an intermediate frequency. The intermediate Hfrequency outputsignalsfrom the amplifier I2! are-applied to the'` primary -circuit o-trans'fermer-I Ss'comprising winding I8and condenserJIl--which is fir-iductivelyA coupled` to `the secondary :circuits consistingwf winding 2| and condenser 2-2;- A rectif-ying circuit comprising the anodev '2`5aridl the cathode 26 Aand-the resistors 3`I and;v 32-fis arranged across the Lsecondary circuit 'tol' provide a series rectificationcircuit for the applied-- input signals;V The recti-iied `voltage developed Afacrossl--resistOr- 32= is impressed upon theiaudio-ampliner39, by means of conductor 38. 'If-his vvre'ctiiied y-voltage, orA detected `signal is then amplified andA applied.- tof the loud speaker 4 I for reproduction-f'. -Atlthe-same 'time'aportion of fthe rectiii'ed--isig'nalY voltage developed across resistor -32Jalsc isappliedto the-describedvautomatic voltage control -networkfor conventional automatic Lvolume control of `the ampliers I 2l `and '-I 4.

The receiver circuitshown-and -described herein is purely illustrative 'and' is not -tosbe considered alimitaticnf The method of --increasing the se- `lectiv-ilty-ofthis detector circuit as will be presently Vdescribedlisrequally applicable *.to other receiver arrangements -such as tuned radio -frequency receivers, orthe like.`

A -It will be notedthat-during the described operation-of '-the receiver,`A the cathode resistor 29 has `beenmaintained -at a 4zero value of resistance, fandathat 'theoperationffotthedetector stage is conventional.. Assume. now--that, under these conditions, la curve -is -plottedwithirespect to the selectivity ot thisreceiveri Reference is no w madetoFig-. l2=of--the drawingv wherein curve 50 may be considered representative of the overall selectivity achieved Vby-`the conventional opera- 'ltionofithe receiverl described. rlfhecurve 50 -shows -thi'si-gnal amplitude withv respect -toresonance, whereby a minimum degree of signal attenuation exists at the-resonance point 5I, and there `is a gradual sloping oft-to maX-imum attenuation on ibcthsidesoffthe resonance pointe AIt is apparent that,as curve 5-deviates-from-the center frequency or resonance pointfthe receiver sensitivity :ffall'sfoi'while the band-pass coverage increases. A'I'l'iiseis -normal behavior in a prior art receiver "aidrepresents the problem to which the present invention is directed.`- Itis also pointed'ou't that, 'with `respect to the -curve 5U', the llegend shown thereon R=0 indicates that this characteristic lrespon'sejcurve is applicable to the receiver during 'thetime' that the `cathode :resistor 29v (R) 'is adjustedto a value of 'zero (0) resistance.

Before describing in 'detail the application of yth'ebuckinlg voltage to the vvdetector stage it will bejunderstood 'that the use of the term voltage jdevel oped'applies to voltage'produced'by a current iow throughan impedance device, such as a resistor," `whereas the term voltage impressed does not neczessarily require suchfa current iiow.

" Reiference is nowjmadetotherectifying circuits "shown in Fig. 1 wherein,V as'previous'ly de"scribed,-1

` of diode24. The primary circuit -appliesga ,radio coA frequency voltage acrossA itsrectier .circuit whereby theflrectied voltage Vis proportional: vto the applied radio -frequencyvoltage- Similarly, the secondary -rectiiiercircuit Eis arrangedv so that its rectied voltagel is proportional to. the -radio frequency voltage applied across the-.secondary circuit. Cathode .26is common to bothL the-primary and secondary circuits; 'Il'ierefore` the bucking voltage will exert a vcontrolling-- influence over the rectication action of the detector stage, whereby the effective characteristicA response -of/the detector may begoverned This action` is accomplished inf-a common circuit whereby the `detector cathode, 'operating-at' a positive `rlpotential, .is` ei-ectivelybiased `towards non-conduction-by-superimposingthereonfapostive bucking potential which at times is of greater vmagnitude than 'the positive cathode potential producedbys'ignal detection.

For the purpose vof generatingthe bucking voltage the 1cathode resistor 29=may5now -be -adjusted-to some value such-as one mego'hm- The cathode 26 and the series resistors-Z-are-'now commune-to the primary-and secondary-rectifying lcircuits 'through the `anodes 46-and-25e-respectively.' This provides an-arrangement-whereby,V if the rect-ined primary, vor bucking, voltage developed across res-istor2'9; is of greater magnitude than the peak of the radio frequencywave as appliedbythe secondaryv toits recti-iiercircuit,A there will ybe no secondary circuit signal rectication. Conversely, if the 'rectified-sec- 'ondary voltage developed'vacross resistor-f2f9'is higherqthan the peakr of the radio frcquencylwave applied by'` the primary circuit A'to vits- Vassocia-ted rectifier circuit, there will -be Ano `rectification of primary circuit energy This controlledrecti- `cation action ofthe diode 24 andits associated circuits may be best described as a teeter-totter effect, whereby the predominance in magnitude rofr-eitherthe vprimary or the Secondary voltage over theV vother causes the predominant voltage, by a blocking action, to assume control over the circuit having the voltage of lesser magnitude. The actual control position achieved is dependent upon the relationshipI of the incoming signal Ato the center frequency of the transformer I9.

The resistance ratio existing between resistors 29 and 4I'determines the magnitude of the buckingtvoltage which is applied to the secondary cir- Clll In resonant circuits it is well known 'that 'the Vmaiimumresponse is produced when there is impressed upon the circuit energy having a fre- `'quency 'which is equal to or closeto the resonant frequency :of the circuit'. When 'two such cir- 'cuits are coupled together as in the case of the `quency indicated by the point59.

velop al voltage. rapplied to the left hand portion of l'the diode 24 ,mary'voltage Consequently, at resonance there -isa voltage of greater magnitude impressed upon the left,v hand, portion of the diode 24 than the voltage impressed upon the rightjhand portion of Vthe diode. Furthermore, the magnitudes of theprixnary winding deviates yonreither side of ,the resonant frequency.y The decreasel in the .magnitudes of the two voltages is gradual, in the case, of both circuits.v However, since the primary winding I8 is more broadly .tuned than the'secondarywinding 2 l, the rate of decrease of the secondary voltage is greater than the rate of decrease of the primary voltage. In the present invention, use is made of this difference in the l.performance of the primary and secondary cirqcuits.

To understand the operation of this feature of the invention` reference now will be made to Fig. 3 of the drawing. In this figure, the curve 54 represents the characteristic response of the primary winding L8 of the intermediate frequency transformer I9. Similiarly, the curve 55 is generally representative of the response characteristic of the secondary winding 2|. It will be noted that, in view of the broader tuning of the primary winding, the curve 54 has a much broader skirt than the curve 55. Also, it will be noted that the maximum amplitude of the curve 55 is considerably greater than the maximum amplitude of the curve 54. It is seen, from an inspection of these curves, that there are critical frequencies at which the primary and secondary voltages are substantially equal. Two such frequencies exist because of the symmetrical shapes of the curves with respect `to the resonant or center frequency. The line 5B indicates the two equal voltage points which occur at critical frequencies 51 and 58 approximately at the same deviation above and below the resonant fre- Consideration now will be given to the performance of the receiver when it is being tuned byv moving upwardly through the frequency spec- `trum such as indicated by moving from left to right on the curves of Fig. 3. Until the frequency represented by the point 6l is reached it may be assumed that there is no response produced in either the primary or secondary circuits. As the receiver is tune-d further toward the frequency indicated by the point 51, the primary circuit begins to respond as indicated by the curve 54. However, at the beginning of the primary response, there is no response effected by the secondary circuit, inasmuch as is assumed that the receiver has not been tuned to a frequency indicated by the point B2. The response nected resistor 29. As the primary circuit voltage increases, the cathode 2S ofthe diode is increased in positive polarity which has the effect of biasing off both portions of the diode toward vthe region of nonconduction.

vAs the receiver tuning progresses to a frequency beyond that indicated by the point B2, the lsecondary circuit beginsto respond to -de- This voltage, however, when is ineffective to produce conduction in thisportion of the diode by reason ,of the fact that, at

Vths'time, there is sufficient lrrectification.`ac-

.trol of the rectified `voltage derived from the primary circuit.

` As the receiver tuning progresses to a frequency which is slightly closer to the resonant frequency than that indicated by the point 41, the secondary circuit voltage which, in this region of the response curve, is increasing at a relatively rapid rate exceeds in magnitude the primary circuit voltage which is increasing at a relatively slow rate. The rectified primary circuit voltage then is not capable of biasing the diode 24 bythe production of a high enough positive potential at the cathode 26 thereof to prevent the left hand portion of the diode from functioning as a signal rectifier. Consequently, signal rectiiication in they secondary circuit begins quite abruptly and the magnitude of the rectified signal voltage is quite substantial, equal approximately to the Voltage referred to at the critical frequency. The

effect in the output circuit of the left hand portion of the diode 24 is the same as if the secondary response characteristicwere represented by the broken line curve 63.

From this point on to resonance and further to the critical frequency 58 beyond resonance where the equal voltages for the primary and secondary circuits again is produced, it is the secondary circuit which controls the operation of the diode.

It is seen, therefore, that in the region of resonance, the cathode 26 will have developed thereat a much greater positive potential by reason of the operation of the left hand portion thereof than is capable of being developed by reason of the primary voltage impressed upon the right hand portion. Thus, it is seen that the right hand portion of the diode is rendered nonconducting when the receiver is tuned near to resonance. It should be particularly noted that the described functioning of the diode is accomplished by means of the connection of the resistor 29 into the cathode circuits of the two portionsof the tube.

The variation of cathode resistor 29 to a particular value causes the resistance ratio between resistor 29 and resistor 41 to be likewise varied ,whereby a similar change is effected in thevoltage distribution circuit across these two resistors. Thus the bucking voltage may be increased by increasing the value of resistor 29 whereby sece5; the critical frequency points may be moved still closer to the center frequency thereby effectively narrowing, to a greater degree, the overall selectivity characteristic of the receiver apparatus. The extent of this narrowing eifect may be had by again referring yto Fig. 2 of the drawings wherein the curves 52 and 53 are indicative of A different operating conditions in the detector circuit as caused by the application of bucking voltagesof different magnitudes. With, respect to curve 52 the cathode resistorrvalue of one. meg;-

asians!) ohm-was employed-fin the detectort'stage, .whereas the curve 53 indicates a still further selectiveiac- -tion resultingfrom the usageoff-acathode resistor ofif2 'meg'ohms g.

Itf willY beapparent from 'the curves shown in Fig. =2 1 that, Awith respect toi rdetector response, the? distance lbetween the' critical frequencies, as representedfby the response: 'skirtsof curves 52 or 'candthe center frequency;` decreases with the application! Ofaliigher value of cathode resistance. Curve 52 pertains -to the-detector response 'operating with a cathodefresistor value' of one -megohm (R=1M)fand is obviously responsive, at itswidestvpoint,"to' a'frequency bandwidth which isfslightl-y g'reater'th'anl-kilocyclesieach side of center frequency. Curve 4:53" represents the receiver' response achieved' with fa' cathode resistor value'of 2 megohms -R`=`2M),'tliis" curve shows thatthe'ioverallreceiver responsehas been considerably sharpened, vwhereby'tlie` band-'width at itswidest-point'is substantiallyless than 5kilo- 4'cycles either side tif-"center frequency. vThe'n'ormal 'characteristic response of the receiver, as illustrated by curve 55| (R) indicates a band width response, which isclearly in 'excess of the relatively narrow-response achiev'edfby the application of Vthe bucking' voltage to the detector cir- 'cuitias indicated bythe-curvesl 52 and '53.

-The apparent increase in lselectivity gained herein provides 'forease of tuning, inasmuch as the carrier signal will yabruptly"appear or disappear asy the tuning elements: pass through resonance. Side band interference and side band noises will also beelimii-'fated `by Athe selective characteristic aga-'ined herein,`-in-co'mparison with a receiver having a`brcad"'characteristic response.

It is to be'understbod that'tlef'einbodiment of this'inv'ention in a radio receiver ydries not alter or aii'ect the 'factual fredi'iency'response :of ythe various 'individual tuned lcircuits contained therein, butrather results only inthe changing of the effective oprating'fregiencies' of the signalenergy existent "across resistor 32.1- Inother words, by'causilfgla curi-"ent flew 'the secondary rec'ztiii'er circuit landfchangingit relative to frequency only"du'r'ingl-thev periodfw-heii the voltage, develcped' in the "secbiidaryftfun'e'df circuitv exceeds that develped in f the 4primary tuned:' ':ircuit, 'a substantially selective tuning effect is" established.

It will be notedfwi'tlir'e'specdtc Fig. 2, that the selective skirt clipping action` herein obtained does not substantially 'aiec't th'ense of the characteristic response curves. Therefore, signal distortion is heldto a minimum and further distortion possibilities are also eliminated due to the fact that, as resonance, the signal voltage developed across resistor 29 isv determined almost entirely -by the flow of current in the secondary circuit. Thus, a modulative component will not be apparent in the output rectified signal which normally would introduce, in-a biased diode detector, a degree of signal distortion. If optimum ratios are also established between the resistors 32 and 41 there will be as low distortion, related to modulation percentage, as in any similar well designed conventional diode circuit. However, if the value chosen for resistor 29 were to be sufficiently high and beyond that value herein disclosed, the nose of the response curve would then be affected since the response curve skirt would be pulled in to such an extent, that a new response curve would be established having characteristics approaching that of the crystal filter stage of a communication receiver.

Referring to the tuned circuits which precede 10 the selective detectorfcircuit vherein disclosed, overcoupling or critical'coupling maybe employed. Critical coupling between the primary andfsecondary circuits provides greaterA sensitivity, together with easeofalignrnent.` The use of overcoupling will produce a characteristic responsecurve having a relatively broad' nose. For broadcast reception ypurposes itis desirable "that theresponse curve Ynose remain relatively broad while, at the-same time, the response curve skirt widthk may be effectively narrowed. Itis again pointed out that ythis result is relatively uncommon, inasmuchv asin normalE receiverl circuit 'operation, the narrowing Vof 7the i response curve skirt usually also results ini vtheoverall sharpening of the response-curve; nose.

It will also be vapparent to those `skilled `in the art that the intermediate frequency transformer I9 .and itsassociated source' 'of signalv energy may be replaced `by other receiverapparatusof tuned radio frequencytype. In' all instances ,and variations such` aslll uned primary circuits, capacitively couple circuits, or permeability tunedcircuits, the applidfsignal'energy Will be rectified and selectively l reduced' in accordance with', the aforementioned description.

By means of "the, selective' detector. circuit, einbodied in the instant invention,'a"high'delity receiver of relatively bra'dtuningcha'racteristics, or a conventionalfhoinereceiver'of average frequency response characteristics; may lbey read'- ily converted intoV a `substaiital'ly: selective "receiving device, the'd'egree bfselecti-vity `being variable, for the separationof` stations that are closely aligned, orfor 'eliminating noisesf'and side band interference. The 'value of th'e presentl invention is further' increased by consideration' that the unusual res'ults f herein obtained'v are* lachieved by the novel application of'a-relativelyfewand inexpensive receiver components.

A further advantage isf' obtained-1 bytheuse 'of but a single control to vary the-'receiverselectivity degree. This same controlmaybe employed to render the selectivity function inoperative, whereby the receiver 'will revert to its vnormal broad response characteristics.

This selective detector circuit lmay be applied with good usage tocommunic'ation type receivers whereby a high degree of apparent selectivity may be obtained'in a'single stage, in comparison withtl'ie plurality of stages, such as intermediate frequency amplifiers now required. The circuit herein disclosed thereby ,permits a substantial reduction to bem'ade inthe number of 'selectivity achieving components'norrnallyemplnyed in a receiver. Inv contrast with the crystal filter devices normally employed in'rcorhinunication receiversa further vimi?)rovernent is noted by the ilse bf 1this invention, y in that vi'ce modulate-d signardoes not loseits-'intlligibility when received under the extremely selective conditions applied by this circuit and, at the same time, there is eliminated the necessity of signal phasing which is an absolute requirement for the operation of a crystal lter stage. It is to be further noted that a substantial saving would be effected by the aforementioned reduction in intermediate frequency amplifier stages and other components. The eiciency of this communication receiver may then be further increased by generally utilizing radio frequency amplifier stages to precede the converter stage, thus raising the overall gain of the receiver and, at the same time, effecting considerable increase in sensitivity and image suppression.

It will also be understood that the cathode resistor 29 maybe replaced by a varistor, thereby securing a somewhat automatic selectivity characteristic, whereby the degree of selectivity would be determined by the magnitude of the input signals. The cathode resistor may also be replaced by atriode control circuit for the same automatically selective purpose, such as is accomplished by the -varistor, or for remote `selectivity control purposes whereby a biasing potential may be variably applied to the triode grid to overcome the intelligence signalrectifying circuit.

While the invention is applicable to a wide choice of component'values which will establish the requisite circuit parameters, there follow specifications of a particular arrangement which provides satisfactory operating conditions.

Condenser 2'!V 100 micromicrofarads Condenser 33 100 micromicrofarads Condenser 34 100 micromicrofarads Condenser 45 6.7 micromicrofarads Resistor 29 3.2 megohms maximum Resistor 3| 47,000 ohms Resistor '32 470,000 ohms Resistor 41 470,000 ohms Transformer I 9 #M55-LGF.)

Automatic Winding Co. Step up-aiding The duo-diode rectifier tube herein disclosed may be replaced by other arrangements such as two separate rectifier tubesv having their cathode circuits connected in common.

While there has been described what, at present, is considered the preferred embodiment of the invention, it will be obvious to those skilled inthe art that various changes and modications may be made therein without departing from 'the invention, and it is'aimed, therefore, in the appended claims to cover all such changes and modiiications as falrwithin the true spirit and scope of the invention.

What is claimed is:

l. A radio signal detector comprising, a source of intelligence signals modulated on Van intermediate frequency wave, aviirst resonant circuit coupled to said signal. source and relatively broadly tuned t saidintermediate frequency, a second resonant circuit inductively coupled to said iirst circuit and relatively narrowly tuned to said intermediate frequency, means coupled to said second circuit for detecting said intelligence signals, means coupled to said iirst circuit for developing a unidirectional voltage representative of the response by said first circuit to said intermediate frequency Wave, and means including an impedance device coupled to said detecting means and to said unidirectional voltage developing means for biasing said detecting means to an inoperative state when the voltage developed in said rst resonant circuit exceeds in magnitude the voltage developed in said second resonant circuit.

2. A radio signal detector comprising, two resonant circuits coupled together inductively and having impressed thereon a wave of predetermined frequency Which is modulated in accordance with an intelligence signal, a rst one of said resonant circuits being tuned to have a relatively broad frequency response characteristic with respect to said predetermined frequency and the second one of said resonant circuits being tuned to have a relatively narrow frequency response characteristic with respect to said predetermined frequency, a diode detector coupled to said second resonant circuit and including a resistor connected in circuit with the cathode of said diode, and means including a rectiiier coupled to the first one of said resonant circuits and to said resistor for developing in said resistor a biasing voltage for said diode detector to maintain said detector in a nonconducting state while the voltage developed in said second resonant circuit is less in magnitude than the voltage developed in said rst resonant circuit.

3. A radio signal detector comprising, a pair of diodes each having an anode and a cathode, a resistor connectible in common to both of said cathodes, an intermediate frequency transformer having primary and secondary windings, said primary winding being broadly tuned to an intermediate frequency and said secondary Winding being narrowly tuned to said intermediate frequency, the turns ratio of said transformer Windings being such as to eiect a voltage step up between said primary and secondary windings, means for coupling said primary winding to a vsource of intermediate frequency wave, means for coupling said secondary winding to a first one of said diodes for effecting signal rectification in the output circuit of said first diode when it is in a conducting state, and means for coupling said primary winding to the second one of said diodes to develop a, cathode biasing voltage in said resistor of a character to maintain said rst diode in a non-conducting state until said secondary winding voltage is substantially equal to said primary winding voltage and to render said rst diode conducting .during times that said secondary winding voltage is of greater magnitude than said primary winding voltage.

MAURICE L. ALEXANDER.

REFERENCES CITED UNITED STATES PATENTS Name u Date Groeneveld May 7, 1940 Number 

